The present invention relates to signal processing techniques to form beams applicable to radar and communication systems. More specifically, the present invention relates to methods and apparatus for introducing phase and time delays in a general class of signals composed of a modulated carriers that are applicable to the formation of directive transmission and reception patterns for arrays of antennas and other types of transducers.
Modem communication, radar, and related systems often utilize arrays of spatially separated antennas, or more generally, transducers, to enhance transmission and reception of signals from particular directions. Arrays are used to produce directive patterns, to place nulls in the directions of interfering signals, and to allow directive arrays to adapt to other environmental effects.
Mathematically, the spatial reception directivity afforded by arrays is accomplished by generating a weighted summation of the signals received by the individual antennas/transducers in the array. The spatial transmission directivity is accomplished by individually weighting a common carrier signal transmitted by each array element. Appropriate weighting can produce tightly confined regions of enhanced transmission and reception referred to as beams. In practice, one of two principle implementations have generally been employed for array beamforming: phased arrays and true time-delay arrays.
In phased arrays, phase-shifting units are inserted between the antennas that constitute the array and a summation junction where the individual signals received by the antennas are combined. The phase-shifting units, or simply phase shifters, produce a selectable phase shift in the transmitted and/or received signals. For narrowband signals, appropriate phase shifts can be found that will allow the formation and steering of a beam to any desired location within the constraints of the array topology. Wideband signals cannot be effectively formed into narrow beams by phased array antenna systems. The frequency range inherent in the wideband signal produces a degradation in the beamwidth and an increase in sidelobes.
Generally, for wideband signals, a true time-delay array is required. In a true time-delay array, a variable time-delay unit is inserted between the antennas and the common junction. Ideally the time-delay unit exactly matches the time delays needed to launch a beam at a desired angle and for reception, cancels the differential time delay of the signal incident on the array. By appropriate selection of the time delay imposed on each antenna signal, a beam can be steered to any desired angular location within the constraints of the array topology. Implementation of true time-delay arrays, or more particularly, the required time-delay units, often results in a system that is prohibitively expensive and/or complex.
Consider a signal s(t) 14 emitted by a distant source and incident with angle .theta. on an array of spatially separated antennas 0, 1, 2, . . . , N distributed along a baseline 10 such that each antenna is separated by a constant distance D as illustrated in FIG. 1. Because the signal propagates with a finite velocity, it will in general arrive at the various antennas in the array at different times. It is this time difference of arrival that makes possible beamforming with an array.
A wavefront 12 of signal s(t) defined as s(t=0) will arrive at each antenna within the array at a different time, The signal received by the n'th antenna is given by EQU s.sub.n (t)=S(t-.tau..sub.n) (1)
where the signal delay .pi..sub.n is given by ##EQU1##
where D is the element spacing and c is the signal propagation velocity in the transmission medium.
The output of the antenna array without appropriate compensating delays in the antenna signals is given by ##EQU2##
as the summation of the signals received by the individual elements. Except for a few trivial cases, this summation alone will yield very poor results. What is needed is a way to align the signals received by the various paths so that they combine in an advantageous way. The optimum solution is to introduce a time delay .tau.'.sub.n in each path such that the total delay is the same for all antennas. If this is done, then a beam in the direction of the received signal is formed.
This is the approach taken by true time-delay arrays. Each antenna in the array is followed by an adjustable time delay element that permits a selectable time delay to be applied individually to each antenna signal. If the sum of the imposed time delay .tau.'.sub.n and the .tau..sub.n given by equation (2) is the same for all antennas, a beam is formed in the direction .theta.. The beam direction is not a function of the frequency.
While the implementation of the time-delay unit is beyond the scope of this discussion, generally it involves the introduction of variable-length transmission paths between the antennas and the common junction. Various approaches including switched waveguides, photonic delay lines, multiple reflection cavities, and quasi-optical combining have been proposed. Historically, time-delay units have been difficult or impractical to implement.
As an alternative, narrowband implementations utilizing phase shifters have found greater application. If the incident signal s(t) is made up of a single frequency it is represented mathematically by EQU s(t)=e.sup.j.omega.t (4)
where .omega. is the angular frequency. In this case, the received signal for the n'th antenna is given by ##EQU3##
Thus, in the single-frequency case, the time delay is replaced by a phase shift. Beam forming and steering can now be effectively accomplished by simply introducing a phase-shifting element after each antenna. By selecting a phase shift corresponding to EQU .phi..sub.n =(.omega./c)D sin .theta. (6)
a beam can be generated in the direction of .theta.. A variety of phase shifters are known in the art, and numerous examples of phased-array antenna systems are available.
While this approach can be used even when the signal s(t) is is not comprised of a single frequency, degradation does occur in the beam-forming performance. As the frequency varies from .omega., the beam is seen to shift or squint. Since the beam produced for a broadband signal is the summation of the individual beams associated with the individual frequency components that make up the beam, a phased array used with broadband signals produces a distorted beam with higher sidelobes.
A final method for beamforming is the use of purely digital beamforming. This method can be viewed as a digital true time-delay beamforming method. In digital beamforming, the received signals are converted from analog to digital and a digital means such as a shift register is used to introduce the required delay. The problems with purely digital beamforming is the need for excessively high-performance digital components such as high speed A/D converters. Of chief concern is the need to sample the received signal at a rate of about three times the carrier frequency. If the signal is not sampled at a high enough rate, the information in the signal will not be preserved by the digital samples. A similar problem arises in the analog-delay method when the delay is produced by a time-delay means having discrete delay times.
Therefore, it would be desirable to have a broadband beamforming apparatus that did not suffer from the serious beam squint and beam broadening of the phase-array antenna but which did not have the complexity or cost of either the analog or digital true time-delay approach. A beamforming method and apparatus for transmitting and receiving arrays such as disclosed here will overcome a longstanding problem in the area of broadband beamforming on high gain (highly directive) arrays for modulated carrier signals.